This invention relates to an electrographic developing method, and more particularly to a method of developing latent electrostatic images to toner images with a magnetic brush comprising a magnetic toner.
Latent electrostatic images formed on the surface of a support (such as electrostatic recording paper in facsimile or a photoconductive member in electrophotographic copiers) can be developed to visible images advantageously, for example, by the method disclosed in U.S. Pat. No. 3,909,258 in which the image bearing surface is brushed with a magnetic brush of low-resistivity magnetic toner particles prepared from a dispersion of fine magnetic particles in a resin and having carbon black or a like electroconductive material embedded in the surfaces of the toner particles.
Because of various advantages, this method has found wide application in electrophotographic copies, etc. However, the copiers in which this method is used have been limited solely to those of the Fax type in which the copy paper itself is developed. In fact, the method still remains to be improved for use in so-called plain paper copiers (PPC) which appear to be ideal copying machines.
Presumably this is due to the fact that whereas the magnetic toner used for this method must be highly amenable to the injection of charges and therefore have a low resistivity, extreme difficulty is encountered in transferring such a toner of low resistivity from the image bearing surface to the copy paper. Stated more specifically, when latent images developed with the low-resistivity toner, namely toner images, are to be transferred from the support to paper by corona charging with the use of the usual transfer unit, the toner image becomes markedly broken during the transfer operation, thus affording a copy which is no longer suited for use. The break in the toner image appears attributable to the phenomenon that when the toner is exposed to corona charges, toner particles are transferred to an area of the paper other than the image area to be formed thereon, or some toner particles are scattered due to a change in polarity.
Various attempts have been made to give the magnetic toner the highest possible resistivity and to thereby eliminate the problem involved in the transfer of the magnetic toner by a corona discharge unit, but the remedy, although achieved to some extent, adversely affects the development of the latent image, thus failing to provide useful toner images.
Additionally the method disclosed in U.S. Pat. No. 3,909,258 has another drawback in that it is low in the reproducibility of gradation when used in a PPC for developing latent electrostatic images having a high potential of up to about 500 to about 1000 V, even if the copies obtained are usable.
This will be described below in detail with reference to FIG. 1, which shows the relationship between the potential of latent electrostatic images and the amount of magnetic toner deposited on the image bearing surface when the latent image is developed by the method of the above-mentioned U.S. patent. The potential P is plotted as the abscissa vs. the amount T as the ordinate. The solid line A in FIG. 1 represents the above relationship as established by the use of a magnetic toner having a low resistivity of 10.sup.8 ohm-cm (commercial magnetic toner presently available for use in Fax-type copiers). The dot-and-dash line B shows the relationship as determined by the use of a magnetic toner having a high resistivity of 10.sup.-- ohm-cm. The solid line A indicates that when the potential of the latent image is in the range of 0 to 500 V, the potential is approximately proportional to the amount of deposited toner, permitting satisfactory reproduction of gradation within the range, whereas at potentials above 500 V, the amount of toner remains substantially constant irrespective of the potential, failing to reproduce the gradation. The dot-and-dash line B reveals that latent images, if having a potential of about 1000 V, can be developed only to a low overall density with a lesser amount of toner deposition.